Recently, in vehicles equipped with an internal combustion engine, it has been known to perform cylinder cut-off operation to halt fuel injection for a part of cylinders in a relatively low load condition and to operate the internal combustion engine with the remaining cylinders for the benefit of improving fuel consumption of the internal combustion engine (for example, Japanese laid open patent publication No. JP 3-85340.)
However, as shown in FIG. 2B, during the cylinder cut-off operation, a maximum internal pressure P2 of a halted cylinder which is halted from combustion by halting fuel injection becomes considerably lower than a maximum internal pressure of a combustion cylinder which is performed fuel injection. Consequently, the difference ΔP (=P1−P2) of the maximum internal pressures between the combustion cylinder and the halted cylinder becomes larger (FIG. 4). Therefore, it causes the problem that the difference of the generated torques between the combustion cylinder and the halted cylinder becomes larger, and the torque of the crank shaft fluctuates during the cylinder cut-off operation, and the rotational fluctuation becomes larger, and then, vibration of the internal combustion engine becomes larger.
To overcome the above mentioned problem, it is proposed that intake valves of halted cylinders are maintained at a closed position in all strokes during the cylinder cut-off operation and exhaust valves of the halted cylinders are opened in intake stroke and exhaust stroke and are closed in compression stroke and expansion stroke so that exhaust gas is emitted from the halted cylinders through the exhaust ports after the gas is sucked into the halted cylinders through the exhaust ports, compressed and expanded in the halted cylinders.
In this way, with sucking and compressing the exhaust gas whose pressure is higher than intake air, in the halted cylinders during the cylinder cut-off operation, the maximum internal pressures of the halted cylinders P2 are maximized and the difference ΔP (=P1−P2) of the maximum internal pressures between the combustion cylinder and the halted cylinder is minimized, for reducing vibration of the internal combustion engine during the cylinder cut-off operation (for example, Japanese laid open patent publication No. JP 2001-132484.)
However, in accordance with a recent study of the inventors of this invention, it was found that it was not the best way just to maximize the maximum internal pressure P2 of the halted cylinder for reducing vibration of the internal combustion engine effectively during the cylinder cut-off operation, and that when the maximum value of a combined waveform of internal pressures of all of the halted cylinders at that time and the maximum internal pressure of the combustion cylinder P1 were balanced, vibration of the internal combustion engine would be effectively reduced during the cylinder cut-off operation.
For example, as shown in FIG. 5, in the case that the engine was an in-line four-cylinder engine, the number of the halted cylinder was two cylinders during the cylinder cut-off operation and a maximum internal pressure of #1 cylinder and a maximum internal pressure of #4 cylinder were equal, it was found that vibration of the internal combustion engine would be effectively reduced during the cylinder cut-off operation when a total value of the maximum internal pressures P2 of the two halted cylinders at that time and the maximum internal pressure P1 were equal (when formula P1=P2×2 was satisfied).
Furthermore, the combustion pressure of the combustion cylinder changes in accordance with change of operating state of the internal combustion engine during the cylinder cut-off operation, and the maximum internal pressure P1 of the combustion cylinder changes. As a result, a proper maximum internal pressure P2 of the halted cylinder, which is needed to reduce vibration of the internal combustion engine, changes. The number of the halted cylinders during the cylinder cut-off operation also changes the proper maximum internal pressure P2 of the halted cylinder.
However, in the Japanese laid open patent publication No. JP 2001-132484, the control pattern of the exhaust valves of the halted cylinders is not changed even if the operating state of the internal combustion engine (the combustion pressure of the combustion cylinder) changed during the cylinder cut-off operation. Consequently, it is not capable of changing the maximum internal pressure of the halted cylinder purposefully even if the operating state of the internal combustion engine and/or the combustion pressure of the combustion cylinder changed. Therefore, depending on the operating state of the internal combustion engine during cylinder cut-off operation, it can be occurred that the difference between the maximum internal pressure P1 of the combustion cylinder and the total value of the maximum internal pressures of all of the halted cylinders becomes larger. Consequently, vibration of the internal combustion engine becomes larger, and as a result, stable vibration-reduction effect is not obtained.